One of the biggest challenges in semiconductor packaging is picking, handling and processing of very thin semiconductor chips. The semiconductor chips are generally provided on a carrier tape, which generally contains a whole semiconductor wafer that has been cut into small chips, a process frequently referred to as dicing. The semiconductor chips are thus commonly referred to as dies. The carrier tape is often the same tape that supported the wafer during dicing, often referred to as dicing tape. Die thicknesses of down to 25 μm are common today, and there is an ongoing trend to decrease thicknesses even further. Thicknesses of 15 μm have already been anticipated on semiconductor manufacturer roadmaps.
Die bonders pick a single die from the carrier tape and place and subsequently attach the picked die onto a substrate or onto another die. In most situations, the die is attached permanently, but configurations where dies are only attached temporarily also exist. Often, a temporary attachment is subsequently turned into a permanent one by an additional heating and/or pressing process. Typically, thin dies come with a wafer backside lamination (WBL) or film over wire (FOW) lamination, i.e. an adhesive film which is applied to an unstructured side of the wafer or die. A process of thinning wafers, applying an adhesive film to the wafers, mounting the wafers to a carrier tape and frame, and dicing them into individual chips is usually referred to as wafer preparation. The lamination, which allows for the dies to be attached due to its adhesive properties, may be provided either between the carrier tape and the wafer, or on a surface of the wafer facing away from the carrier tape.
Picking, placing, and transport between pick and place locations may be carried out by a single chip handling unit in the die bonder. In modern die bonders, however, a plurality of chip handling units is often present: In general, picking takes place from a so called wafer table, and is assisted by a zeroth chip handling unit, also referred to as die ejector. The die ejector facilitates the removal of the die from the carrier tape, e.g. by pushing the die against a first chip handling unit called pick unit—from underneath the carrier tape. The pick unit subsequently picks the die from the carrier tape in a pick process and hands it over to a second chip handling unit, also referred to as place unit. The place unit places the die onto a target position, where it is attached. In some cases, at least one third chip handling unit—a so called transfer unit—is provided to hand the die from the pick unit to the place unit. An example is given in WO 07118511 A1 which is hereby incorporated by reference in its entirety. In this manner, a die may be attached to a substrate, e.g. a leadframe, printed circuit board, multilayer board etc., or to another die, which itself may be have been attached in the same way.
The fabrication of very thin wafers and the corresponding dies is very expensive as compared to standard thickness wafers without lamination. Sawing, picking as well as handling of very thin dies have significant yield losses. Most of the yield is lost during wafer preparation, die picking, or subsequent handling, resulting in damaged dies due to typical defects as e.g. broken dies, cracked dies, chipped dies, etc. Both place and attachment processes, in comparison, are more reliable, giving rise to only negligible loss.
In particular for stacked die bond processes where two or more dies are attached onto one another, attaching of a broken die may have dramatic consequences: If a damaged die is attached onto a stack, all previously attached dies in that stack—also referred to as package—are lost. In an extreme situation, for example, a package consisting of 15 stacked dies may thus be lost by attaching a broken 16th die on top of it. Assuming a pick process yield of 99%, an expected package yield drops to 0.99^16, i.e. to 85% in this example, and to only 44% for a pick process yield of 95%.
Known inspection methods for detecting broken, cracked or chipped dies on die bonders are performed before the pick process—which itself has limited yield—and are generally based on imaging a die surface under surface illumination, followed by image processing. These inspection techniques have limited reliability due to low crack contrast and crack width, die warpage—also referred to as potato chip effect—and due to the interference of crack signatures with other similar looking patterns on the die surface.
Until recently, die bonders have not offered any mechanism or functionality to detect die cracks prior to the bond process and after the pick process. There were two main reasons for that: First, the typical “direct” pick & place architecture of die bonding machines does not allow for the usage of special (pick & place) tooling which incorporates means for die crack detection. Second, die cracks are hard to detect as their appearance varies extremely, both in shape, width etc.
In WO2011/018375 A1, a method and apparatus for inspecting a chip prior to bonding by means of an optical crack detection method was described.